03 IRS: Speed sensitive 'clunking'

[Booksix]

Ok, the quick rundown. Got an 03 Cobra IRS from ebay for my 88 Thunderbird. The owner had 3.90's in it but said he'd put 3.55's back in it for me; he also said it was just rebuilt with a 'master rebuild kit'. The guy claimed he worked at a big speed shop, outfitting cars for drag racing and that he sold the IRS units for customers when they got into the range of too much power for them to be efficient, cost wise. But, I think it was BS because when I picked up the IRS it was sitting in a carport at his apt complex. So, I'm guessing he had no clue what he was doing when he put the 3.55's back in.

So, the clunk. I haven't gone above 20 yet but I have a speed sensitive clunk from the rear-end (can't tell if it's more on one side or not) and I'm just wondering if anyone could list all the possibilities; gears or otherwise.

 

[Uncle Meat]

Well I can tell you that stock 03/04 Cobra's came straight from the factory with plenty of drive line slack which resulted in clunking. It's the nature of the beast so to say. The IRS is made of of more components than a SRA so there's naturally more slack to begin with. If you're driving along at a steady speed and you just lift your foot off the throttle, there was a noticeable "clunk" as the drive line unwound. Once you placed your foot back on the throttle it would clunk again when it took the slack back up.

Is your clunk like what I've described?

U.M.

 

[Booksix]

No, it's not slack, at least not in that manner. It's constant, not just when torque is applied or removed. As soon as the car starts moving, it begins at a regular interval and speeds up as the car speeds up.

 

[Uncle Meat]

Time to yank the cover, perform a visual for obvious damage, and then if necessary start measuring the tolerances to see if it was assembled correctly. Sounds like something is definitely out of whack.

U.M.

 

[Booksix]

Yeah. I'm guessing he pulled the Motive gears and put the Ford gears back in without changing any shims. This fricken sucks. I have to move to a new house about 15 miles way in two days and I really needed to drive the car to get it there...

Also, here's a video of what it sounds like (not sure if mine does it in reverse):

YouTube - pinion noise


edit: could I swap the carrier and gears from my 8.8 straight axle into the IRS if there is already damage?

 

[SVT32VDOHC]

Two simple things.

1. Some Cobras came with 12mm bolts from the factory, but the holes in the bushings accept 14mm bolts. Did he give you bolts with it? Did you use stock Mustang/T-bird bolts from the 80's? If so, they could be only 12mm.

2. The rear IRS bushing at the rear diff cover, rips. That causes clunking too. Check that, and replace it with poly bushings. One for the rear and two for the front.